The present invention provides a hot melt conductor paste composition that is particularly useful for forming electrically conductive patterns in photovoltaic cells.
Conventional conductive pastes used in the electronic materials market are liquids at room temperature. Such pastes typically consist of conductive powders or flakes dispersed in a liquid vehicle. Such pastes are applied to substrates by conventional screen printing methods, pad printing, extrusion, dispensing, or other conventional application methods, which are well known.
One of the problems associated with the use of such conventional conductive pastes is that such pastes must be forced dried, typically in an oven, before additional electronic material pastes can be applied and/or before any additional processing operations can be performed on the component being formed. The forced drying step takes time and energy, and can result in the release of volatile organic compounds. Furthermore, the repeated handling of the components during drying operations can lead to component breakage.
Magrini et al., U.S. Pat. No.4,243,710, disclose a thermoplastic electrode ink for use in the manufacture of ceramic multi-layer capacitors. The thermoplastic electrode ink composition according to Magrini et al. includes finely divided ternary (Ag/Pt/Pd or Pt/Pd/Au) or binary (Ag/Pd) metal powders dispersed in a thermoplastic medium that is a solid at room temperature. Upon heating to a temperature between 100xc2x0 F. to 250xc2x0 F., the thermoplastic electrode ink composition according to Magrini et al. melts to form a flowable ink that can be applied by screen printing to sheets of dielectric ware, which are then laminated into multi-layer capacitors without a drying step.
It would be highly advantageous to use a thermoplastic electrode ink composition such as disclosed in Magrini et al. in applications other than the fabrication of multi-layer ceramic capacitors. Unfortunately, however, an electrode ink composition according to Magrini et al. will not sufficiently adhere to certain substrates such as single crystal silicon and polycrystalline silicon (hereinafter simply silicon), which are commonly used in the fabrication of photovoltaic cells and the like. Furthermore, such thermoplastic electrode ink compositions will not meet the electrical requirements needed in such applications.
The present invention provides a conductive composition that is a solid at room temperature (xcx9c25xc2x0 C.), but which melts at a temperature of from about 35xc2x0 C. to about 90xc2x0 C. to form a flowable liquid that can be applied to a silicon substrate by screen printing. The composition according to the invention does not need to be force dried after application because it rapidly solidifies and adheres to the substrate after printing. The solidified screen printed pattern can withstand rigorous part handling without detaching from the substrate or becoming dented or broken. Subsequent electronic material paste layers can be immediately applied directly over the solidified screen printed pattern.
The hot melt conductor paste composition according to the invention comprises conductive particles and glass particles dispersed in a thermoplastic polymer system. The conductive particles preferably comprise either silver or aluminum, and can be in the form of powders and/or flakes. The glass particles preferably comprise one or more glass frits having the following composition by weight: from about 60% to about 95% PbO, up to about 30% SiO2, up to about 15% B2O3, up to about 10% Al2O3, up to about 10% ZrO2, up to about 5% P2O5, and up to about 5% RuO2. The thermoplastic polymer system preferably comprises a blend of from about 50% to about 90% by weight of one or more fatty alcohols, from about 5% to about 20% by weight of one or more cellulose ethers, and optionally up to about 10% by weight wood rosin and/or soy lecithin. The composition also preferably comprises at least about 0.01% by weight up to about 5% by weight of one or more C12 or higher saturated fatty acids.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.